#SBus client

## Overview

Well... Not much to say here. I have the radio, the receiver, a Teensy3.1 and some spare time. Let's hook everything together =D


## Protocol

### Technical details

The technical details originaly come from __Uwe Gartmann__ from the MBed website (source link in the Links section). Because it would have been to easy to use standard settings, SBus serial communications use:

* A baudrate of 100000 b/s
* An inverted logic
* One parity bit
* Two stop bits
* MSB first (that the only common thing !)

Luckily, the Teensy3 can manage that =)

The SBus protocol sends a message every 7ms (high speed mode) or every 14ms (low speed mode).

### SBus protocol

A common SBus message requires 25 bytes of data:

    F0 xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx yy 00


* F0 is the start byte. It should always be 0xF0. You can sync on that I guess if you want to.
* xx are the data bytes. There should be 22 of them. SBus manages 16 analog channels, and uses 11bits for each, so 11*16/8 => 22 bytes.
  * CH1 uses the first data byte (let's call it D0), and the 3 Most Significant Bits (MSB) of D1
  * CH2 uses the 5 Least Significant Bits (LSB) of D1, and the 6 MSB of D2
  * And so on... CH3 uses the 2 LSB  of D2, the whole D3, and the MSB of D4...See, its not that easy to process (12bits would have been too simple ^^ )...

* yy is the flag byte
  * Bit 7 (0x80) is CH17, it's a digital/binary channel.
  * Bit 6 (0x40) is CH18, same.
  * Bit 5 (0x20) indicates a __Lost Frame__
  * Bit 4 (0x10) indicates __Failsafe mode__
  * Bits 3-0 are not used and sould be 0
* 00 is the stop byte. It should always be 0x00. You can also do some check to check there was no lost/added byte in the serial communication.

### Typical Frame

    F0 2E 90 FB 1F 1D EC 8E 0F B6 F3 DE 07 C6 A0 D4 1A 83 50 6A 0D 41 A8 00 00


* Starts with 0xF0
* Flag is 0x00, no error.
* Ends with 0x00

### Radio off

If the receiver was getting data and then looses the connection (eg, if you turn off the transmitter). It outputs something like this:

    F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 30 00 
    
The only interesting byte is the flag byte; 0x30 means the receiver is in failsafe mode and that a frame was lost (no kidding :-°). Good =)

## Code

Well, it's heavily adapted from the [[:rc:dev:sport_master|SmartPort Master code]]. Same warnings, same wiring (Gnd on Gnd, Vcc on Vcc, SBus signal on TX1/PB17). It doesn't do much, but might help start/debug another project...

Here's the last version of the code: {{:rc:dev:sbus:tsysbusrecv_20141008_1942.tar.gz|}}

## Next ?

* Decode the CH* values
* Port to Attiny (cost, embeddable, ...)
* Emulates a joystick with Teensy3, so I can use the RC simulator softwares without any cable.

## Links

* http://forum.pjrc.com/threads/23956-Teensy-3-UART-settings
* http://developer.mbed.org/users/Digixx/notebook/futaba-s-bus-controlled-by-mbed/